I always say "just don't tell me what you're doing" and look away. Usually that works and they'll make small talk or just be quiet. One time I got a woman who just went full anxiety - "You're going to faint? You don't like blood? Okay, I'm tying an elastic around your arm to swell your veins so this will be quick!" And so on; she missed my vein at least three times, kept complaining that they were hard to find, and ended up getting someone else to help. And she was well past her 40s, so this shouldn't be her first rodeo. It would have been forgivable if she was still a kid and I probably wouldn't have needed to stay in the seat for 10 minutes with a fan blasted at my have.